Exploring Hydraulic Residence in Minnesota`s Sentinel Lakes

Exploring Hydraulic Residence in Minnesota`s Sentinel Lakes

Exploring Hydraulic Residence in Minnesota’s Sentinel Lakes: Implications for
Management
Lee Engel & Joe Magner
MPCA & University of Minnesota
Abstract
Lake systems present a challenge in determining how water pollutants and associated solutes
cycle over time. Lake hydraulic residence time is dependent on several factors including:
volume, watershed size, location within a watershed and climatic variability. The use of the
stable isotopes of hydrogen (Deuterium expressed as δD) and oxygen (δ18O) can provide some
hydrologic insight and management guidance for the development of a Total Maximum Daily
Load (TMDL). Analyzing the stable isotopic composition of lake water δD and δ18O over time
can aid in identifying source water input mixing and evaporative processes, further explaining
the relationship between hydraulic residence time and pollutant loading. δD and δ18O were
compared to the isotopic composition of atmospheric water vapor which has a known isotopic
concentration at specific latitudes and air temperatures (Burns and McDonnell, 1998; Dansgaard,
1964). The deviation in amplitudes of the fractionation of lake water to water vapor was modeled
to predict hydraulic residence time for each lake. Twenty-four lakes throughout Minnesota were
sampled over a three year period and residence times were calculated. Each of the twenty-four
lakes is part of the “Sustaining Lakes in a Changing Environment (SLICE)” project, a study led
by the Minnesota Department of Natural Resources.
Introduction
The landscape of the state of Minnesota is rich in water resources as a result of the last
glaciation. The Minnesota lakes region is one of the largest concentrations of freshwater lakes in
the United States. The state of Minnesota’s history, culture, and economic vitality revolve around
these unique water resources. Water-based tourism and outdoor recreation such as fishing,
swimming, and boating along with lakeshore residential development are important activities
that drive many local economies in Minnesota. Clean water lakes not only provide many
recreational and economic opportunities to residents and tourists, but also provide excellent fish
and wildlife habitat. Drinking water is also a limited, but important component derived from
surface waters.
In order to correctly assess the condition of water quality and properly manage each lake type in
Minnesota, deep or shallow, by Major Land Use Region (MLUR), it is critical that managers
have a wide range of tools and techniques at their disposal. The use of basic physical and
chemical water quality parameters provide insight into lake conditions, but used alone limit
knowledge and the ability to manage effectively. These physical and chemical parameters merely
provide a snap shot of what lake water quality is like at the time of sampling, however, when
baseline water quality data is supplemented with varying modeling techniques (watershed,
bathymetric, and lake cycling) the true condition of a lake can be better understood.
Some lakes are more or less resilient to water quality impairments because of their morphology
and hydraulic residence time. Since lake basin morphology is relatively constant, hydraulic
residence time should be explored in detail. Currently, lake hydraulic residence time is estimated
based on apparent input and output; typically excluding groundwater exchange. This presents
many challenges in understanding how hydraulic residence time varies depending on seen and
unseen water source inputs/outputs. Surface water bodies present a challenge in determining how
water cycles through them. Wetlands, streams, and lakes obtain a water budget from various
sources including precipitation, surface water and/or groundwater (Magner and Alexander,
2008). Depending on the location of the water body, evaporation rates can drastically influence
water budgets as well. Analyzing the stable isotopic composition of lake water δD and δ18O can
provide insight into mixing and evaporative processes.
Background
Isotopes are atoms of a specific element with a different mass due to a different number of
neutrons. Water is composed of oxygen and hydrogen isotopes which occur at different
frequencies throughout the hydrologic cycle and are influenced by temperature and latitude
(Dansgaard, 1964). Over time, water droplets move within the hydrologic cycle and slight
changes in the composition can be measured. This causes individual water bodies and endmember source waters to have their own isotopic signature (International Atomic Energy
Agency, 2009; Craig 1961b). The use of stable isotopes has emerged as a vital tool for defining
water sources in lakes and predicting hydraulic residence time. The study of isotopes has allowed
scientists to determine hydraulic residence time without extremely long data sets. Previously, this
technique has been used to quantify the contribution of groundwater, precipitation, and runoff or
snow melt to specific bodies of water (Burns and McDonnell, 1998; Magner and Alexander,
2008; Fritz and Fontes, 1980, 1986). The ratio of stable oxygen (δ18O), and hydrogen (δD) in a
given lake is dependent on a variety of physical processes that is reflected in the evaporative
signature of end-member sources and the final mixing zone. The decoupling of δD and δ18O
occurs due to fractionation; deuterium evaporates slightly faster than δ18O (Craig, 1961a). The
relationship between atmospheric δD and δ18O shows a linear correlation which is known as the
Meteoric Water Line (MWL). The isotopic enrichments, relative to ocean water, display a linear
correlation over the entire range for waters which have not undergone excessive evaporation
(Craig, 1961a). Water bodies often show deviations from the MWL, referred to as the
“evaporative line,” as a result of local climate and water budgets. For a given water sample,
fractionation can produce a unique isotopic signature that can offer hydrologic insight. It is
difficult, however to estimate the magnitude of end-member source waters that contribute to the
overall budget of a lake. Lake residence time will be driven by lake morphology, evaporation
rates, and flow into and out of a lake. By identifying the seasonal isotopic signature and its
oscillations, compared to that of latitudinal water vapor throughout the open water season, it is
possible to predict hydraulic residence time (Burns and McDonnell, 1998). The difference in
fractionation between the isotopic water vapor and that of a water body directly correlates to
residence time. This difference can be quantified and modeled to give an estimated hydraulic
residence time for a specific lake.
Freshwater lakes present the challenge of having many different water sources contributing to
their annual water budget. Quantifying hydraulic residence time in lakes via stable isotopes will
offer new insight that may explain many other lake water quality metrics. It may also be used to
further explain which lakes are more or less susceptible to perturbations in the watershed that
lead to lake impairment. Hydraulic residence time within lakes has a direct effect on the
concentration of nutrients, contaminants, and other chemicals of concern. The only way to
determine residence time through water chemistry is to look at the isotopic fractionation of a lake
and determine the seasonal oscillation in δ18O and δD (Maloszewski et al., 1983; Pearce et al.,
1986; Stewart and McDonnell, 1991; Burns and McDonnell, 1998). This type of data will
provide residence time estimates for a small collection cost. It will allow for better lake models
to be developed and aid in the understanding of how pollutants cycle. Hydraulic residence time
can also be used as a tool to predict lakes that may be at risk of accelerated eutrophication.
Methods
Stable isotope samples, δD and δ18O, were collected from 24 lakes throughout the state of
Minnesota (Figure 1). Currently, these 24 lakes are part of the “Sustaining Lakes in a Changing
Environment” (SLICE) project, a joint study conducted by the Minnesota Department of Natural
Resources and the Minnesota Pollution Control Agency to address climate change on
Minnesota’s lakes (Valley, 2009). The SLICE study will provide many physical and chemical
lake water parameters that can be used in conjunction with stable isotopes δD and δ18O to assess
overall lake condition and explain lake hydrology. These lakes represent typical lakes found
within Minnesota’s four MLURs; Canadian Shield, Transition Forest, Glacial Drift and Northern
Forest, and Prairie and Cornbelt. Study lakes range from deep oligotrophic lakes with high
groundwater contributions to shallow hypereutrophic, runoff driven lakes. These lakes have very
different water budgets and reflect a range of lakes systems found throughout the upper Midwest.
Valuable insight should be gained into the contributions of source waters and the impacts of
pollutants in MN Lakes.
Figure 1. Sentinel Lakes
Over the course of a three year period (2008-2010), each of the 24 lakes were visited during the
months of May, July, and October in order to collect stable isotope samples. Sampling periods
were timed to correspond closely with spring and fall turnover and during mid-summer when
evaporation is highest. By sampling at these key time periods, seasonal variations in lake isotopic
compositions for each lake were captured. To ensure that a representative sample was taken, a
composite of the top two meters of lake water were taken over the deepest part of the lake basin.
An integrated water quality sampler was used for sample collection, which is a polyvinyl
chloride tube 2 meters in length with an inside diameter of 3.2 centimeters. This eliminates large
variations from water sources and precipitation events entering the lake within a short duration of
sampling. Surface water sampling protocols were followed from the MPCA’s Standard
Operating Procedure for Lake Water Quality Sampling (Anderson and Lindon, 2009). Samples
collected were placed in 125ml plastic water quality bottles, sealed and sent to the University of
Minnesota Biometeorology Lab in the Department of Soil, Water, and Climate for analysis. All
liquid water samples were analyzed for their isotopic composition using a laser spec-troscopy
system (Liquid Water Analyzer, DLT-100, Los Gatos Research, Inc) coupled to an autosampler
(HT-300A, HTA) for simultaneous measurements of δD and δ18O.
Stable isotope compositions were compared to the isotopic composition of atmospheric water
vapor which has a known isotopic concentration at specific latitudes and air temperatures (Burns
and McDonnell, 1998; Dansgaard, 1964). The deviation in amplitudes of the fractionation of lake
water to water vapor was modeled to predict hydraulic residence time for each lake.
Estimates for seasonal δ18O water vapor values were determined based on seasonal mean
minimum and maximum air temperatures using equation 1 (Yurtsever in 1989 notes).
(1)
δ18O water vapor = (0.521+/-0.014) T – (14.96+/-0.21)
Where T is the air temperature in degrees Celsius.
Air temperature data used in Equation 1 was derived from the Minnesota State Climatology
Working Group web page run by the State Climatology Office, Minnesota Department of
Natural Resources. Minimum and maximum seasonal air temperatures were calculated to
represent the expected range of stable isotope compositions of atmospheric water vapor for each
sentinel lake. Air temperatures were calculated by sentinel lake location using ArcMap10.1 for
each season; December-February, March-May, June-August, and September-November. The
maximum seasonal range of calculated atmospheric stable isotope concentrations was modeled
with observed lake water isotopic concentrations to estimate residence time.
Hydraulic residence time was estimated using Equation 2 (Maloszewski et al., 1983) by
comparing the amplitude of a best-fit curve for precipitation to the amplitude of a similar curve
for the water of interest. Seasonal changes in the δ18O composition of precipitation at temperate
latitudes tend to follow a sinusoidal pattern. This pattern occurs over one year, reflecting the
seasonal changes in tropospheric temperature. Measured changes in δ18O composition for a
stream, lake, pond, soil water, or groundwater are obtained for a given location during different
seasons. Mean hydraulic residence can then be calculated if the seasonal waters are considered in
steady state and well mixed reservoir with an exponential distribution of residence time as:
-1
(2)
2
1/2
τ = ω [(A/B) – 1]
Where τ is the estimated hydraulic residence time, in days, ω the angular frequency of variation
(2/365 days) or (0.07172), A the input amplitude, and B the output amplitude.
Results
Regional Trends
Throughout the state of Minnesota regional trends in δ18O and δD were similar in all four MLRUs studied
(Figures 2 and 3). A general transition to heavier isotopic concentrations exists from north to south in
Minnesota lakes. The most pronounced difference occurs in the Canadian Shield lakes. This is a result of
climatic conditions that are unique to the Canadian Shield MLUR. Weather systems in this region often
originate in the Arctic, resulting in light isotopic sources of water vapor as compared to weather systems
originating in the Gulf of Mexico. Lake water budgets may also receive higher volumes of light isotopic
snow melt runoff than lakes south of the Canadian Shield. A transition to heavier isotopic concentrations
is evident along the Glacial Drift Northern Forest MLUR. Here δ18O and δD values are slightly heavier
than Canadian Shield values. The heaviest isotopic compositions were found in the Transition Forest and
Prairie and Corn Belt MLURs. Heavier δ18O and δD values are a result of higher annual evaporation rates
and higher annual mean temperatures.
Study lakes show a strong correlation, R2 = 0.950, to an evaporative line in Minnesota (Figure 4).
Deviations from the MWL are evident at a range of scales. Results were interpreted by state, MLURs, and
by individual lake. The further along the evaporative line a lake plots, the heavier the isotopic
composition. As a result, Canadian Shield lakes plotted lowest on the evaporative line representing a
lighter isotopic composition than the other three MLUR’s studied.
18
Figure 2. δ O values for study lakes within the MLUR’s.
Figure 3. δD values for lakes within the MLUR’s.
18
Figure 4. MLUR’s δ O vs. δD compared to the MWL.
Canadian Shield
Stable isotope δ18O concentrations in the Canadian Shield are lighter than in the other three major land
use regions (Figure 2). A wide range of lake types were sampled in the Canadian Shield MLUR.
Measured δ18O compositions for each lake in the region are shown in Figure 5. Canadian Shield
lakes showed a relatively strong correlation, R2 = 0.773, to the evaporative line (Figure 6). This
correlation may have been stronger in absence of White Iron Lake. Since, the lake is a reservoir
with a very short residence time and plotted near the MWL. Deep oligotrophic lakes, Bearhead and
Trout, had the smallest δ18O amplitude in the Canadian Shield MLUR. As a result, these two lakes had the
longest calculated hydraulic residence times (Table 1). Echo, Tait, and Elephant lakes are relatively
shallow and are well connected to their watersheds through many tributaries. As a result residence times
were shorter because direct flow pathways move water into and out of these lakes (Table 1). White Iron
Lake had the shortest residence time of all the Canadian Shield lakes because this lake has a large
watershed and is an impoundment on the Kawishiwi River (Table 1). Large volumes of water move
through White Iron resulting is a very short residence time.
Annual climate variation also has an effect on δ18O amplitudes and residence times. In 2010, δ18O
amplitudes were reduced because of dryer climatic conditions as compared to 2008 and 2009. As a result,
residence times increased in most Canadian Shield lakes.
18
Figure 5. δ O values for study lakes within the Canadian Shield MLUR.
18
Figure 6. Canadian Shield δ O vs. δD compared to the MWL.
Table 1. Canadian Shield lakes modeled residence time ranges based on seasonal mean minimum and
maximum air temperatures.
Min.
2008
Max.
2008
Min.
2009
Max.
2009
Min.
2010
Max.
2010
Range
(Yr)
Residence Time
(Yr)
Bearhead
2.5
2.6
3.9
3.8
17.7
17.3
15.2
2.5-17.7
Echo
0.7
0.7
2.8*
2.7*
2.2
2.1
1.5
0.7-2.2
Elephant
3.2
3.3
3.2
3.2
5.6
5.5
2.4
3.2-5.6
-
-
1.8
1.8
1.6
1.7
0.2
1.6-1.8
Trout
6.9
7.7
8.7
9.3
10.5
11.8
4.9
6.9-11.8
White Iron
1.0
1.0
0.8
0.8
1.3
1.3
0.5
0.8-1.3
Lake Name
Tait
* Values omitted because of missing data.
Bearhead Lake
Bearhead Lake had the heaviest isotopic composition of the six Canadian Shield lakes sampled with δ18O
values ranging from -5.3 to -6.3 per mil. Differences in residence time calculations are explained by lake
basin morphology, climate, and lake levels. Bearhead Lake has a small watershed with no surface water
inlets or outlets limiting, the annual water budget to direct precipitation and diffuse wetland and
groundwater flow. Bearhead Lake appears to have a very low sill, during wetter than normal climates,
2008-2009. Lake levels remained above the sill allowing water to diffuse out of the lake basin through
surrounding wetlands adjacent to the lake’s southern basin. This resulted in a calculated residence time of
2.5-3.9 years. In 2010, dry climatic conditions caused lake levels to decrease below the sill of the lake
basin inhibiting outward flow. As a result, calculated lake residence time increased to 17.3-17.7 years
(Table 1). This fluctuation in residence time is reflected in the observed amplitude of δ18O values during
wet and dry years. In wetter years, 2008 and 2009, δ18O amplitudes were much greater than in 2010. Dry
conditions and inhibited lake water movement greatly reduced δ18O amplitudes in 2010.
Echo Lake
Echo Lake’s isotopic composition shows high annual variability with δ18O values ranging from -11.2 to 7.6 per mil. This range can be explained by the connectivity of Echo Lake’s watershed to the lake itself
and its small lake volume. Echo Lake is large, but very shallow, with a surface area of 461 hectares and a
maximum depth of 3 meters. Five inlets act as conduits directing precipitation that falls within the
watershed to the lake in a relatively short period of time. The Echo River serves as the lake’s outlet.
Residence time calculations are relatively short, 0.7-2.2 years (Table 1). 2008 was a wetter year than
2010, resulting in more frequent flushing of the lake water. The 2009 spring sampling event was not
collected, therefore the maximum isotopic range and residence time were not calculated. However,
summer and fall 2009 δ18O values were similar to 2008 δ18O values, suggesting that residence time in
2009 was probably closer to 2008’s calculation then 2010’s (Table. 1).
Elephant Lake
Elephant Lake’s δ18O values range from -7.6 to -6.1 per mil. Annual δ18O amplitudes are similar in 2008
and 2009. In 2010, the δ18O amplitude decreased slightly resulting in a higher residence time calculation.
Residence time calculations ranged from 3.2-5.6 years (Table 1). Isotopic concentrations show strong
correlation from year to year, plotting on the evaporative line. Strong correlation to the evaporative line is
likely a result of the lakes small watershed and limited connectivity. These morphometric characteristics
limit the probability of large δ18O amplitude shifts caused by runoff or evaporation. Elephant Lake has
four small inlets and one lake outlet via Elephant Creek.
Tait Lake
Tait Lake’s δ18O values range from -9.5 to -7.3 per mil. Tait Lake was added to the Sentinel Lakes
program in 2009. Therefore, isotope samples were not collected in 2008. Isotope values in 2009 poorly
correlate and plot above the MWL. It is unclear what would cause this to happen. However, 2010 shows
good correlation to the evaporative line resulting in a residence time calculation of 1.6-1.7 years (Table
1).
Trout Lake
Trout Lake’s δ18O isotopic composition varied the least of the six Canadian Shield lakes. Conservative
δ18O values are strongly influenced by lake morphology, primarily depth and lake surface area to volume
ratio. Trout Lake is the deepest of all the Canadian Shield lakes sampled with a maximum depth of 77 ft.
and a mean depth of 35 ft. A large volume to surface area ratio limits evaporative influences on annual
δ18O amplitudes. Trout Lake’s δ18O values ranged from -8.0 to -8.6 per mil. and residence time
calculations ranged from 6.9-11.8 years (Table 1). Trout Lake’s δ18O amplitudes decreased each year,
2008-2010, as climatic conditions in the area became progressively dryer and limited water entering from
Marsh Lake and exiting through the Kadunce River. This reduction in source waters, snow melt, and
precipitation decreased variation in δ18O amplitudes and increased residence time in Trout Lake.
White Iron
White Iron Lake’s δ18O isotopic composition varied the most of the six Canadian Shield lakes (Figure 5).
The wide range of δ18O values is not surprising since White Iron Lake is a reservoir with a very large
watershed. The contributing watershed has a total area of over 592,000 acres (931 square miles). White Iron
Lake’s δ18O values ranged from -6.5 to -11.6 per mil. and residence time calculations ranged from 0.8-1.3
years (Table 1). Isotopic δ18O values show good correlation to the MWL which suggests that White Iron’s
lake water consists of primarily unaltered precipitation. This means that there is little storage in the
watershed and water flows quickly through the White Iron Lake system.
Glacial Drift Northern Forest
A wide range of lake types were sampled in the Glacial Drift Northern Forest MLUR. Measured δ18O
compositions for each lake in the region are shown in Figure 7.The isotopic compositions of all
lakes sampled in the Glacial Drift Northern Forest MLUR show a very strong correlation to the
evaporative line, R2 = 0.974 (Figure 8). In general, a shift towards lighter isotopic concentrations was
observed from 2008 to 2010. This correlates to precipitation patterns in the region which were dryer in
2008 and became progressively wetter by 2010.
Many lakes in this MLUR have important groundwater contributions because of glacial depositional
sediment. Elk and Ten Mile lakes are deep and known to have high groundwater contributions. As a
result, these two lakes had the highest calculated residence times (Table 2). Portage and Red Sand Lakes
have similar morphology and watershed characteristics but have the heaviest and lightest isotopic
composition of all Glacial Drift Northern Forest Lakes. This suggests that source waters for the two lakes
are different.
Red Sand Lake is much further south than Portage Lake and may receive heavier isotopic precipitation
from storms originating in the Gulf of Mexico. It is likely that Portage Lake receives lighter isotopic
precipitation from arctic storms and receives higher snowmelt contributions in the spring. Hill Lake has
two distinct basins. Isotope samples were collected in the deeper northern basin resulting in higher
residence times as compared to what would likely be observed in the shallow basin of Hill Lake.
18
Figure 7. δ O values for study lakes within the Glacial Drift Northern Forest MLUR.
18
Figure 8. Glacial Drift Northern Forest δ O vs. δD compared to the MWL.
Table 2. Glacial Drift Northern Forest lakes modeled residence time ranges based on seasonal mean
minimum and maximum air temperatures.
Min.
2008
Max.
2008
Min.
2009
Max.
2009
Min.
2010
Max.
2010
Range
(Yr)
Residence Time
(Yr)
Elk
6.2
6.4
10.9
10.5
2.9
2.9
8.1
2.9-10.9
Hill
7.7
8.2
2.0
2.0
9.9
10.1
8.1
2.0-10.1
Portage
2.0
2.0
3.4
3.3
2.3
2.3
1.5
2.0-3.4
Red Sand
1.3
1.4
1.4
1.3
1.6
1.7
0.4
1.3-1.7
South Twin
2.5
2.6
6.4
6.2
5.6
5.7
4.0
2.5-6.4
Ten Mile
3.8
3.9
15.7
15.2
22.6
22.9
19.1
3.8-22.9
Lake Name
* Values omitted because of missing data.
Elk Lake
18
Elk Lake had variable δ O and δD concentrations resulting from a large watershed, depth, and many
different source water inputs. Large contributions of groundwater flow along with many diffuse wetlands
and tributaries contribute to Elk Lake’s annual water budget. Elk Lake’s δ18O values ranged from -5.9 to 7.5 per mil. and residence time calculations ranged from 2.9-10.9 years (Table 2). Residence time and
δ18O values are strongly influenced by the amount of precipitation in a given year. 2008 and 2010 were
both wetter than normal years which increased lake flushing and reduced residence times. In 2009,
precipitation values were normal and a residence time of 10.9 years was calculated. Increased residence
time in 2009 is likely related to reduced outflow to Lake Itasca which forms the headwaters of the
Mississippi River.
Hill Lake
Hill Lake has two distinct lake basins; the north basin is deep and stratifies during summer months while
the southern basin is relatively shallow and acts as the lakes outlet. Isotope samples were taken from the
deep northern basin. Hill Lake’s δ18O values range from -6.8 to -8.1 per mil. and residence time
calculations ranged from 2.0-10.1 years (Table 2). Annual δ18O amplitudes were similar in 2008 and
2010. In 2009, the δ18O amplitude was much greater resulting in a lower residence time calculation.
Portage Lake
Portage Lake had the lightest isotopic composition of the six Glacial Drift Northern Forest lakes sampled,
with δ18O values ranging from -6.7 to -8.9 per mil. Residence time calculations ranged from 2.0-3.4 years
(Table 2). Short residence times are expected in a shallow lake with a 17 ft. maximum depth and mean
depth of 7.5 ft. Source water inputs from the watershed, which is primarily forested, are consistent and
supported by similar annual δ18O amplitudes. Slight shifts in isotope concentrations occurred along the
evaporative line which can be attributed to seasonal variability in precipitation and evaporation. Portage
Lake’s isotopic concentration shifts from light water in the spring to increasingly heavier water though
the summer and fall.
Red Sand Lake
Red Sand Lake had the heaviest isotopic composition of the six Glacial Drift Northern Forest lakes
sampled with δ18O values ranging from -1.6 to -5.1 per mil. Annual δ18O values show the strongest
correlation with the evaporative line as well. Red Sand Lake is the southernmost lake in the Glacial Drift
Northern Forest land type region. As a result, Red Sand Lake likely has higher evaporation rates and
receives heavier isotopic precipitation, rain vs. snow, than other lakes in this region. Residence time
calculations ranged from 1.3-1.7 years (Table 2). Short residence times are to be expected in a shallow
lake with a water control structure. Residence times and δ18O amplitudes vary slightly depending on
annual source water contributions and lake volume change.
South Twin Lake
South Twin Lake’s δ18O values ranged from -4.6 to -6.5 per mil. and residence time calculations ranged
from 2.5-6.4 years (Table 2). Annual precipitation and evaporation fluctuations affect δ18O amplitudes
and how they plot on the evaporative line. Higher evaporation rates and limited source water
contributions caused isotope concentrations to shift toward heavier δ18O values in 2008. Lighter δ18O
values were observed in 2009 and 2010 during wetter years. Lake water became progressively lighter as
precipitation increased, suggesting that snow melt or arctic derived water vapor are contributing to annual
lake water budgets. This seems to be supported by South Twin Lake’s geographic location which is the
furthest Northwestern lake in the Glacial Drift Northern Forest region.
Ten Mile Lake
Ten Mile Lake’s δ18O values ranged from -3.7 to -4.7 per mil. and residence time calculations ranged
from 3.8-22.9 years (Table 2). Residence times in Ten Mile Lake show the highest variability of all lakes
in the Glacial Drift Northern Forest region. The lake is very complex with a maximum depth of 208 ft.
and a mean depth of 53 ft. Groundwater is also known to make up a large portion of the lakes annual
water budget. As precipitation values increased form 2008-2010, δ18O amplitudes decreased and
residence times increased. This suggests that wetter years are influenced by lighter isotopic snow melt and
arctic weather systems may have a large influence on annual water budgets
Transition Forest
A wide range of lake types were sampled in the Transition Forest MLUR. Measured δ18O compositions
for each lake in the region are shown in Figure 9. The isotopic composition of lakes sampled in this
region show a strong correlation to the evaporative line, R2 = 0.961 (Figure 10). In general, a shift
towards lighter isotopic concentrations was observed from 2008 to 2010. Lakes in the western portion of
the Transition Forest region received an annual surplus of 8-12 in. of precipitation in 2010. Lakes without
an outlet, such as Belle, showed a shift to a lighter isotopic composition that was more pronounced. This
may result in an over estimate of residence time since lake water is being displaced and lake levels have
not stabilized. Carlos, Cedar, and South Center are all deep lakes, however their annual water budgets are
different. Carlos and South Center are both part of a chain of lakes. During dry years, 2008-2009,
discharge from Lake Carlos decreased and residence time increased (Table 3). South Center Lake has a
relatively short residence time for a deep lake due to complex hydrology. During drought years water
levels can drop relatively fast. In addition, a groundwater sink in the lake bed increases annual lake water
loss. Pearl and Peltier Lakes may receive large watershed contributions during wet periods but residence
times remain relatively short. Shallow depth and limited lake volume allow for quick flushing of lake
water.
18
Figure 9. δ O values for study lakes within the Transition Forest MLUR.
18
Figure 10. Transition Forest δ O vs. δD compared to the MWL.
Table 3. Transition Forest lakes modeled residence time ranges based on seasonal mean minimum and
maximum air temperatures.
Lake Name
Min.
2008
Max.
2008
Min.
2009
Max.
2009
Min.
2010
Max.
2010
Range
(Yr)
Residence Time
(Yr)
Belle
3.9
4.2
5.4
5.4
8.4
9.0
5.0
3.9-9.0
Carlos
13.6
14.2
5.1
5.0
4.8
5.1
9.3
4.8-14.2
Cedar
2.6
2.8
8.5
8.3
3.1
3.3
5.8
2.6-8.5
Pearl
1.3
1.4
2.4
2.4
1.8
1.9
1.1
1.3-2.4
Peltier
-
-
0.9
0.9
1.5
1.5
0.6
0.9-1.5
2.7
2.9
2.0
2.0
6.3
6.4
4.5
2.0-6.4
South Center
* Values omitted because of missing data.
Belle Lake
Belle Lake’s δ18O values ranged from -3.4 to -1.5 per mil. and residence time calculations ranged from
3.9-9.0 years (Table 3). Isotopic δ18O values parallel the MWL which, suggest that Belle Lake’s water
budget consists of primarily unaltered precipitation. A near parallel shift in the regression of δ18O values,
evaporative line from the MWL, suggests source waters represent a localized MWL influenced by local
climate. In 2008 and 2009 residence time calculations ranged from 3.4-5.9 years as compared to 2010,
8.4-9.0 years. The increase in residence time is a result of 2010 being an extremely wet year causing the
annual lake water budget to consist primarily of precipitation based source waters. This caused the δ18O
amplitude to decrease resulting in a higher residence time calculation in 2010. In addition precipitation
based source waters also caused a shift to a much lighter isotopic composition.
Lake Carlos
Lake Carlos δ18O values ranged from -4.7 to -3.5 per mil. and residence time calculations ranged from
4.8-14.2 years (Table 3). Lake Carlos is the last lake in the Alexandria, MN chain of lakes and its outlet
forms the Long Prairie River. The lake is deep with a maximum depth of 160 ft. and a mean depth of
45.7ft. 2008 residence time calculations were the greatest, 13.6-14.2 years, because of low lake levels and
reduced discharge out of the lake. This reduced the δ18O amplitude causing residence time to increase. In
2009 and 2010, δ18O amplitudes were similar. Increased lake levels and flow rates reduced residence time
ranges to 4.8-5.1 years.
Cedar Lake
Cedar Lake’s δ18O values ranged from -6.1 to -4.6 per mil. and residence time calculations ranged from
2.6-8.5 years (Table 3). The lake is deep, with a maximum depth of 88 ft. and a mean depth of 37 ft.
Cedar Lake’s watershed is relatively small and source waters likely consist of groundwater and direct
precipitation. In 2008 and 2010, δ18O amplitudes were fairly consistent. However, a reduction in δ18O
amplitude was observed in 2009 which caused residence time to increase. The reduction in δ18O
amplitude for 2009 was caused by high fall precipitation rates which skewed Cedar Lake’s isotopic
composition towards the MWL.
Pearl Lake
Pearl Lake’s δ18O values ranged from -7.7 to -3.9 per mil. and residence time calculations ranged from
1.3-2.4 years (Table 3). Pearl Lake is a simple circular basin of moderate depth and has an inlet and outlet
via Mill Creek. 2010 precipitation values were approximately 12 in. higher than normal. This caused a
slight shift in Pearl Lake towards a lighter isotopic composition. Pearl Lake, lies on top of an outwash
plain and is suspected to have significant groundwater contributions, increased precipitation did not have
a significant effect on residence time. Further investigation into surface and groundwater interactions will
occur on Pearl Lake in the future.
Peltier Lake
Peltier Lake’s δ18O values ranged from -8.3 to -4.7 per mil. and residence time calculations ranged from
0.9-1.5 years (Table 3). Peltier Lake is part of a large wetland complex that is relatively shallow and a
dam is in place to maintain water levels. Only one isotope sample was collected in the spring of 2008, so
residence times were not calculated. Residence times for 2009 and 2010 are both relatively short which is
expected in a shallow reservoir.
South Center Lake
South Center Lake’s δ18O values ranged from -3.1 to -1.8 per mil. and residence time calculations ranged
from 2.0-6.4 years (Table 3). South Center Lake is part of the Chisago, MN chain of lakes. The complex
hydrology of South Center Lake makes it difficult to interpret isotopic patterns. The lake has a deep
complex basin, is known to be a groundwater sink, and has had highly variable precipitation and water
levels. As a result, higher resolution and spatial isotope data is needed to interpret South Center Lake’s
hydrology.
Prairie and Cornbelt
18
A wide range of lake types were sampled in the Prairie and Cornbelt MLUR. Measured δ O
compositions for each lake in the region are shown in Figure 11. The isotopic composition of lakes
sampled in this region show a strong correlation to the evaporative line (Figure 12). In general, a shift
towards lighter isotopic concentrations was observed from 2008 to 2010. This shift is driven by
precipitation patterns in the region. In 2008 and 2009, precipitation levels were below normal. 2010 was a
wet year and lakes in this region received 13-20 in. of precipitation above normal. As a result, lake
waters were reflective of the change in source water inputs and a shift to a lighter δ18O composition
was observed. However, annual δ18O amplitudes increased in some lakes and decreased in others
depending on lake outflow and the affect of evaporation in shallow lake basins.
Carrie Lake’s isotopic composition showed the least deviation as a result of above normal precipitation.
This is likely because of limited inflow or runoff to the lake which allows precipitation to infiltrate and
enter the lake via groundwater. Other lakes have well connected watersheds which quickly transport
water from the watershed to the lake itself. These lakes tend to have fluctuating isotopic compositions.
Wet years cause large inputs of source waters with similar isotopic compositions to enter the lake. This
resulted in a shift to lighter isotopic concentrations and an increase in residence times in Shaokotan,
Madison, and St. Olaf. This was not the case in Artichoke or St. James. Wet years did shift to lighter
isotopic concentrations, but large evaporative losses to these shallow lakes increased annual δ18O
amplitudes and decreased residence times. Artichoke’s isotopic composition also parallels the MWL,
suggesting that the lake is influenced by its own local climate and that source waters are likely direct
precipitation.
18
Figure 11. δ O values for study lakes within the Transition Forest MLUR.
18
Figure 12. Prairie and Cornbelt δ O vs. δD compared to the MWL.
Table 4. Prairie and Cornbelt lakes modeled residence time ranges based on seasonal mean minimum and
maximum air temperatures.
Min.
2008
Max.
2008
Min.
2009
Max.
2009
Min.
2010
Max.
2010
Range
(Yr)
Residence Time
(Yr)
Artichoke
5.0
5.2
3.3
3.3
1.9
2.1
3.3
1.9-5.2
Carrie
1.3
1.4
1.4
1.4
2.4
2.6
1.3
1.3-2.6
Madison
2.4
2.6
2.9
3.0
6.7
7.1
4.7
2.4-7.1
Shaokotan
1.4
1.5
5.8*
5.9*
3.8
4.1
2.8
1.4-4.1
1.3
1.4
1.5
1.5
1.1
1.2
0.4
1.1-1.5
17.5*
19.0*
3.2
3.2
5.7
5.9
2.7
3.2-5.9
Lake Name
St James
St Olaf
* Values omitted because of missing data.
Artichoke Lake
Artichoke Lake’s δ18O values ranged from -6.4 to -2.5 per mil. and residence time calculations ranged
from 1.9-5.2 years (Table 4). The lake is relatively shallow and has a small outlet. Isotopic δ18O values
parallel the MWL which suggests that Artichoke Lake’s water consists of primarily unaltered
precipitation. Deviation from the MWL suggests source waters represent a localized MWL influenced by
local climate. Variations along the evaporative line are caused by differences in source water
contributions. This is likely a reflection of source waters during a given year (snow vs. rain). In
general, 2008-2009 had slightly higher than normal precipitation levels and 2010 was a wet year.
As a result 2010 had the lightest isotope values and the largest δ18O amplitude.
Carrie Lake
Carrie Lake’s δ18O values ranged from -8.0 to -5.0 per mil. and residence time calculations ranged from
1.3-2.6 years (Table 4). Carrie Lake is relatively deep and has significant groundwater contributions. The
lake has one outlet which drains through a diffuse wetland into Lake Elizabeth. 2008 and 2009 were both
slightly below average for precipitation and residence time ranged between 1.3-1.4 years. 2010 was a very
wet year, receiving approximately 14 in. above normal precipitation. As a result, summer and fall δ18O
values deviated from the evaporative line and plotted closer to the MWL. This shift shows that 2010’s
annual water budget consisted of a different ratio of source waters, primarily rain water, as compared to
the annual water budget of 2008 and 2009.
Madison Lake
Madison Lake’s δ18O values ranged from -1.8 to -4.3 per mil. and residence time calculations ranged from
2.4 -7.1 years (Table 4). Madison Lake is deep with a maximum depth of 58 ft. and a mean depth of 10 ft.
The lake has an outlet through a culvert to Mud Lake. In addition, strong groundwater influences are
likely based on the local geology. 2008 and 2009 were both below normal precipitation levels and
residence times were between 2.4 - 3.0 years. 2010 was a wet year, receiving approximately 16 in. of
precipitation above normal, and residence times increased to 6.7 - 7.1 years. The 2010 water budget
consisted of a different ratio of source waters, primarily rain water, as compared to the annual water
budget of 2008 and 2009. As a result Madison Lake shifted to a lighter δ18O composition in 2010.
Lake Shaokotan
Lake Shaokotan’s δ18O values ranged from -1.9 to -6.1 per mil. and residence time calculations ranged
from 1.4 -4.1 years (Table 4). One sample was not collected in spring 2009, therefore residence time was
not calculated for that year. Annual precipitation values were highly variable for the years sampled. 2008
and 2009 were both below annual mean precipitation values. Precipitation values in 2010 were
approximately 20 in. above normal. Large volumes of rain event source waters entered Lake Shaokotan
and shifted the lake to a lighter δ18O composition and increased residence time in 2010.
St. James Lake
St. James Lake’s δ18O values ranged from -2.2 to -7.7 per mil. and residence time calculations ranged
from 1.1 -1.5 years (Table 4). St. James Lake is a shallow impoundment on the St. James River. Since the
lake is shallow and has an outlet, residence times are short. Annual precipitation values were highly
variable for the years sampled. 2008 and 2009 were both below annual mean precipitation values and
precipitation values in 2010 were approximately 18 in. above normal. As a result St. James had the largest
range of δ18O values in the Prairie and Cornbelt region. Drainage of the St. James River causes the
watershed to be highly connected to the lake. As a result, lake waters are reflective of source
water inputs which shifted to a lighter δ18O composition in 2010.
St. Olaf Lake
St. Olaf Lake’s δ18O values ranged from -1.9 to -3.5 per mil. and residence time calculations ranged from
3.2 -5.9 years (Table 4). One isotope sampling event was missed in the fall of 2008, therefore, residence
times were not calculated for that year. St. Olaf Lake is relatively deep, 30 ft. for a small lake and has a
watershed to surface area ratio of 3:1. These characteristics limit source water inputs and as a result, St.
Olaf Lake has one of the heavier δ18O compositions of the Prairie and Cornbelt region. 2008 and 2009
were both below annual mean precipitation values. Precipitation values in 2010 were approximately 13 in.
above normal. This caused a shift towards a lighter δ18O composition in 2010.
Discussion
Hydraulic pathways and lake water budgets are complex and variable in Minnesota lakes. Examination of
δ18O and δD provide insight into lake and watershed processes. Once these processes are identified, lake
management strategies can be developed. A variety of factors have been found to influence δ18O and δD
concentrations. These factors include: climate, watershed size, connectivity, and source water
contributions.
Broad climatic patterns are responsible for regional differences in δ18O and δD. As a result, a transition to
lighter isotopic concentrations at higher latitudes was observed throughout Minnesota and in all four
MLURs. Differences in weather pattern origin and air temperature are responsible for the transition. This
transition was expected and serves as the local meteoric waterline for Minnesota, however, in order to
make lake management decisions δ18O and δD compositions must be observed at a finer scale. By
studying δ18O and δD within a specific lake and its watershed, climatic differences from local weather
and its impact on hydraulic residence time can be identified.
Three lakes in the study, White Iron, Artichoke, and Belle had isotopic concentrations that were parallel
to the MWL suggesting that water budgets in these three lakes consist of primarily unaltered precipitation.
White Iron Lake’s δ18O and δD values overlie the MWL, which is expected in a reservoir with a short
residence time. Artichoke and Belle Lake’s isotopic composition plots to the right of the MWL,
suggesting a meteoric waterline influenced by localized climate specific to each lake.
Annual variations in precipitation were found to have significant impacts on δ18O and δD compositions.
The most dramatic changes were observed in 2010 in southwestern Minnesota in portions of the
Transition Forest and Prairie and Cornbelt MLURs, due to 13-20 in. above normal precipitation. This
caused a dramatic shift to lighter isotopic compositions, however residence times increased in some lakes
and decreased in others. Belle Lake experienced the largest shift to a lighter isotopic composition and an
increase in residence time as a result. This suggests that climate may be able to alter hydraulic residence
time during wet and dry periods, but that it is not the most important factor. What is of more interest to
lake managers is how a watershed transports large volumes of water into a lake.
Watershed size and connectivity have a large influence on hydraulic residence time, which varied greatly
among studied lakes. Two statistical outliers were identified and associated with White Iron and Echo
Lakes. Both lakes outliers were found to be a result of watershed size and connectivity. These lakes act
as flow though systems with short residence times. White Iron Lake is reservoir with the largest
watershed studied and Echo Lake is shallow with many tributaries and an outlet through the Echo River.
As a result, large precipitation and runoff in these watersheds can have a significant affect on isotopic
compositions. Both lakes had large δ18O and δD amplitudes resulting in short residence times. Lakes with
small watersheds and limited connectivity, Trout and St. Olaf, had short δ18O and δD amplitudes and the
longest residence times.
Lakes receive various source water inputs, consisting of precipitation and groundwater. Of these,
precipitation in the watershed and inflow through tributaries can be measured. Groundwater presents a
challenge because diffuse seepages around the periphery of the lakes are difficult to quantify. This is
problematic in lakes such as Ten Mile and Elk where, groundwater contributions account for large
portions of the annual water budget. By using δ18O and δD amplitudes, a snap shot of all source water
contributions, including groundwater, are accounted for. This was found to be important in annual
hydraulic residence time variations in Bearhead and Carrie. Bearhead Lake’s hydraulic residence time
showed substantial variation between wet and dry years. Carrie Lake’s watershed has the ability to
infiltrate precipitation, buffering the affects of significantly higher than normal precipitation, opposed to
other study lakes in 2010. Over time, shifts in δ18O and δD amplitudes point to lakes that have a
conservative isotopic composition. These are often deep or have high connectivity through under lying
lake bed sediments where groundwater discharge occurs such as in Ten Mile and Pearl.
Conclusion
Hydraulic residence time in lakes is dynamic; changing as a result of variations in annual source water
contributions and watershed characteristics. Source water contributions affect annual δ18O and δD
amplitudes and are determined by properties such as lake morphology, watershed size, connectivity,
geology, and climate. Stable isotopes δD and δ18O provide insight into annual fluctuations of lake water
budgets and residence times. By documenting how individual lakes react over time to annual fluctuations,
managers can better understand how to protect and remediate lakes and their watersheds.
Using stable isotopes δD and δ18O it is possible to identify annual and seasonal variations in source waters
and residence time while relating them to their watersheds potential for contributing pollutants.
Watershed and lake models use hydraulic residence time to determine how long pollutants may reside in a
lake. Typically, residence time is calculated by using lake volume and balancing inflows with outflows of
a lake. This method does not represent the true range of residence times that exist. Groundwater is usually
not considered and lakes are assumed to be in a steady state which is unlikely with current climatic
variability. This is problematic since many protection and restoration plans are based on model results.
Through the use of isotopes, δD and δ18O, a better understanding of hydraulic residence time and source
water contributions may be understood. Determining what source water contributions are present and how
seasonal variations affect the isotopic concentration is key in determining loading to a lake. Residence
time calculations via stable isotopes δD and δ18O are made with all water sources to a lake being included.
This will in turn enhance lake water quality models and improve management of lakes and their
watersheds.
More work is needed in order to better understand the dynamics of hydraulic residence time and lake
water budgets. An increase in isotopic data will allow for a more precise calculation of residence time
ranges and provide insight into how lakes function under varying climate. When looking at stable
isotopes δD and δ18O in lake it is critical to capture the entire seasonal amplitude. Additional isotopic
work in groundwater is needed in order to fully understand residence time and water budgets.
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